1. Technical Field
The present invention relates to a sealed battery such as a lithium ion battery or alkali battery, and more particularly to a method for manufacturing a sealed battery in which a metal case houses an electrode group that has a portion with the positive electrode substrates exposed formed at one end, and a portion with the negative electrode substrates exposed formed at the other end.
2. Related Art
In recent times, lithium secondary batteries with high energy density (Wh/kg) have been developed as power sources used in portable electronic and communication equipment such as mobile telephones, notebook personal computers and small-size video cameras, and as power sources for hybrid vehicles (HEVs) and electric vehicles (EVs). Among such batteries, particular attention is being focused on sealed batteries, which have high volumetric energy density (Wh/l).
Batteries of this kind, an example of which is disclosed in JP-A-7-326336, are manufactured in the following manner. First, positive electrode mixture containing positive electrode active material is applied to positive electrode substrates (normally aluminum foil) to make positive electrode plates, and negative electrode mixture containing negative electrode active material is applied to negative electrode substrates (normally copper foil) to make negative electrode plates. Then the positive electrode plates and negative electrode plates obtained are stacked alternately with separators interposed, forming a flattened electrode group, which is put into a flattened prismatic case, into which nonaqueous electrolyte is poured, resulting in a prismatic battery.
In the flattened electrode group manufactured in the manner described above, as shown in FIG. 4 for example, at one end there are formed positive electrode substrate exposed portions (portions where positive electrode mixture was not applied) 51a extending from positive electrode plates 51, and at the other end there are formed negative electrode substrate exposed portions (portions where negative electrode mixture was not applied) 52a extending from negative electrode plates 52. Subsequently, a positive electrode collecting lead (positive electrode collector) 53 is deposed at the bottom of the positive electrode substrate exposed portions 51a, and an ultrasonic welding device 60 composed of an ultrasonic horn 61 and an ultrasonic transmitter 62 is made ready. Then the ultrasonic horn 61 is pressed against the top of the positive electrode substrate exposed portions 51a, and by causing the ultrasonic transmitter 62 connected to the ultrasonic horn 61 to ultrasonically vibrate, the positive electrode collecting lead 53 is welded to the positive electrode substrate exposed portions 51a. 
Also, a negative electrode collecting lead (negative electrode collector) 54 is deposed at the bottom of the negative electrode substrate exposed portions 52a, and the ultrasonic horn 61 is pressed against the top of the negative electrode substrate exposed portions 52a, and by causing the ultrasonic transmitter 62 connected to the ultrasonic horn 61 to ultrasonically vibrate, the negative electrode collecting lead 54 is welded to the negative electrode substrate exposed portions 52a. In this way, an electrode body 50 is obtained in which the positive electrode collecting lead 53 and negative electrode collecting lead 54 are welded to the two ends of the flattened electrode group. Then the electrode body 50 thus obtained is put inside a flattened prismatic case, after which the positive lead 53 is welded to a positive terminal portion 55 of a sealing plate, and the negative lead 54 to a negative terminal portion 56 thereof. Next, the sealing plate is welded to the case's mouth, a particular electrolyte is poured through a pour hole formed in the sealing plate, and the pour hole is sealed. Thereupon, manufacture of the prismatic battery is complete.
However, in the ultrasonic joining of the positive electrode substrate exposed portions 61a to the positive electrode collecting lead (positive electrode collector) 53, or the ultrasonic joining of the negative electrode substrate exposed portions 52a to the negative electrode collecting lead (negative electrode collector) 54, it has been necessary, in order to join integrally the layers of each substrate exposed portion to the respective collecting lead, to deliver high outputs of ultrasonic energy according as the number of substrates stacked increases.
But when, in order to render the battery high-output, it has been attempted to increase the number of electrode plates (substrates) stacked, and at the same time to satisfy the joining conditions, there has been the problem that excessive stress is exerted on the join portions and surrounding areas, which fracture due to insufficient strength of the electrode plates (substrates) constituting the electrode body. Particularly in the case of a lithium ion secondary battery, which generally uses aluminum foil and copper foil for the substrates, it has been problematic to obtain a completely joined state without causing damage to the electrode plates (substrates), because such materials are problematic to join.
Accordingly, it was proposed in JP-A-2001-38475 to join the collecting leads to the stack integrally by providing projections in the collecting leads and applying the ultrasonic vibration with the projections superposed over the stack. As FIG. 5 shows, in the method of joining to the stack that was proposed in JP-A-2001-38475, ultrasonic vibration is applied to a stack 71 having, formed at one end, positive electrode substrate exposed portions (portions where positive electrode mixture was not applied) 71a extending from the positive electrode plates, and formed at the other end, negative electrode substrate exposed portions (portions where negative electrode mixture was not applied) 71b extending from the negative electrode plates, and to collecting leads 73, 74 that are superposed over the substrate exposed portions 71a, 71b of the stack 71, so as to fuse the layers of each substrate exposed portion 71a, 71b and join the collecting leads 73 and 74 to the substrate exposed portions 71a and 71b respectively of the stack 71.
Each collecting lead 73, 74 has a protrusion 73a, 74a that projects from the portion that is superposed over the stack 71, and the ultrasonic vibration is applied with the protrusions 73a, 74a pushed against the substrate exposed portions 71a, 71b of the stack 71. In this way, by providing protrusions 73a, 74a on the surfaces 73b, 74b of the collecting leads 73, 74 that join with the substrate exposed portions 71a, 71b of the stack 71, it is contrived to have the ultrasonic amplitude energy of the ultrasonic joining method locally absorbed and diffused, and thus to raise to a high level the efficiency of the ultrasonic joining process. As a result, heat emission, fractures and “bite-in” by the horn in the stack 71's joining portions and surrounding areas are curbed, so that the joining surfaces are rendered high-quality.
However even when, as proposed in JP-A-2001-38475, it has been contrived to have the ultrasonic amplitude energy locally absorbed and diffused by providing protrusions 73a, 74a on the surfaces 73b, 74b of the collecting leads 73, 74 that join with the substrate exposed portions 71a, 71b of the stack 71, there have occurred problems such as rupturing of the electrode plates (substrates) constituting the electrode body, since excessive stress is exerted on the joining portions and surrounding areas. A readily conceivable alternative to the ultrasonic joining method in this case is the laser welding method, which however requires the use of a laser welding machine of high output in order to laser-weld the stack and collecting leads simultaneously, and therefore has resulted in problems such as described below.
Specifically, there has been the problem that since the welding is high-output, it is problematic to shorten the duration (takt time) of the welding process, and therefore the equipment costs increase. There has also been the problem that because it is problematic to shorten the takt time with high output, thermal effects are exerted on the separator and other parts present in the proximity of the welds. Further, there is danger that when the high-output laser beam is shone, welding spatter will be liable to occur, and welding dust could adhere to the electrode body or the interior of the processing equipment, resulting in internal short-circuits or other troubles.